First, the formation of back pressure
In the process of plastic melting and plasticization, the melt continuously moves to the front end of the barrel (measuring chamber), and more and more, gradually forming a pressure, pushing the screw back. In order to prevent the screw from retreating too quickly and ensure that the melt is evenly compacted, it is necessary to provide the screw with a pressure in the opposite direction, and the pressure that prevents the screw from retreating in the opposite direction is called back pressure. Back pressure, also known as plasticizing pressure, is controlled by adjusting the return throttle valve of the injection cylinder. The rear of the injection cylinder of the preplasticizing screw injection molding machine is equipped with a back pressure valve to adjust the speed of oil discharge from the injection cylinder when the screw rotates back, so that the cylinder maintains a certain pressure (as shown in the figure below); The screw backshift speed (resistance) of the full motor is controlled by the AC servo valve.
Second, the benefits of properly adjusting back pressure
1. It can compact the melt in the barrel, increase the density, and improve the stability of the amount of glue, the weight and size of the product. 2. The gas in the melt can be "extruded" to reduce the gas flowers and internal bubbles on the surface of the product, and improve the uniformity of gloss. Slow down the retreat speed of the screw, fully plasticize the melt in the barrel, increase the mixing uniformity of the color powder, masterbatch and melt, and avoid the phenomenon of color mixing of the product. 3. Slow down the retreat speed of the screw, fully plasticize the melt in the barrel, increase the mixing uniformity of the color powder, masterbatch and melt, and avoid the phenomenon of color mixing of the product. 4. Appropriately increase the back pressure, which can improve the shrinkage of the surface of the product and the glue walking around the product. 5. It can increase the temperature of the melt, improve the plasticization quality of the melt, improve the fluidity of the melt when filling the mold, and there is no cold glue line on the surface of the product.
Third, when the back pressure is too low, the following problems are prone to occur
1. When the back pressure is too low, the screw retreats too quickly, and the density of melt flowing into the front end of the barrel is small (loose), and there is more air clamped. 2. It will lead to poor plasticization quality, unstable amount of glue, and large changes in product weight and product size. 3. The surface of the product will appear shrinkage, gas flower, cold material lines, uneven gloss and other undesirable phenomena. 4. Air bubbles are easy to appear inside the product, and the periphery and bone position of the product are easy to walk and not be glued.
Fourth, too high back pressure, prone to the following problems
1. The melt pressure at the front end of the barrel is too high, the material temperature is high, the viscosity decreases, the counterflow of the melt in the screw groove and the leakage flow rate between the barrel and the screw increase will reduce the plasticization efficiency (the amount of material plasticized per unit time).
2. For plastics with poor thermal stability (such as: PVC, POM, etc.) or colorants, thermal decomposition is caused by the increase in the temperature of the melt and the increase of heating time in the barrel, or the degree of coloring of the colorant increases, and the surface color/gloss of the product deteriorates.
3. The back pressure is too high, the screw retreats slowly, and the preplastic refeeding time is long, which will increase the cycle time and lead to a decrease in production efficiency.
4. High back pressure, high melt pressure, nozzle prone to melt glue salivation phenomenon after glue injection, the next time glue injection, the cold material in the water outlet flow channel will block the water outlet or cold material spots appear in the product.
5. In the process of beer molding, due to excessive back pressure, the nozzle leaks, wastes raw materials and causes the heating ring near the nozzle to burn out.
6. The mechanical wear of the premolding mechanism and screw barrel increases.
5. Adjustment of back pressure
The adjustment of injection molding back pressure should be determined by the performance of raw materials, drying conditions, product structure and quality conditions, and the back pressure is generally adjusted at 3-15kg/cm 3. When there is a little air flower, color mixing, shrinkage on the surface of the product and large changes in product size and weight, the back pressure can be appropriately increased. When the nozzle has glue leakage, salivation, overheating and decomposition of the melt, product discoloration and too slow recycling, you can consider appropriately reducing the back pressure.
Back pressure is one of the important parameters to control the quality of melt and product quality in the injection molding process, and the appropriate back pressure plays an important role in improving product quality, which cannot be ignored!
